Musnad Ahmad 5:468

'Abdullah narrated to us, Wahb ibn Baqiyyah al-Wasiti narrated to me, Khalid ibn 'Abdullah informed us, from Husayn, from al-Musayyab ibn 'Abd Khayr, from his father, who said: 'Ali stood up and said: "The best of this nation after its Prophet, may Allah bless him and grant him peace, are Abu Bakr and 'Umar. And we have indeed introduced matters after them, concerning which Allah will decree what He wills."